Nigrosine dye and process for producing the same

ABSTRACT

A non-pyrophoric nigrosine dye and a method of production thereof. A nigrosine dye wherein B−A≦0.04% by weight if, in analysis using a gas chromatograph apparatus equipped with a glass-made sample injection port, the aniline content of the sample determined at a temperature of 200° C. at the sample injection port is written as A% by weight, and the aniline content of the sample determined at a temperature of 260° C. at the sample injection port is written as B% by weight. A method of producing a nigrosine dye which comprises a heat treatment of a neutralized nigrosine base.

TECHNICAL FIELD

The present invention relates to a non-pyrophoric nigrosine dye and amethod of production thereof.

BACKGROUND ART

In 1867, Coupier prepared nigrosine by acting nitrobenzene on aniline inthe presence of hydrochloric acid and iron (Fe) or iron chloride(FeCl₂). Since then, various improvements based on that method have beenmade (Patent Documents 1 to 5).

Despite these improvements in synthetic methods, the nigrosine dyesobtained had a risk of being pyrophoric, i.e. spontaneous ignitionduring storage or transportation.

As a method of suppressing this pyrophoric activity, a techniquebringing a pyrophoric nigrosine base into contact with anoxygen-containing gas in aniline is disclosed in Patent Document 6(Patent Document 7). However, the nigrosine dye obtained by the methoddisclosed in Patent Document 6 (Patent Document 7) was also found to bequite unsatisfactory in terms of conversion to a non-pyrophoric dye.

Patent Document 1: US Patent Publication No. 1896244

Patent Document 2: US Patent Publication No. 1988499

Patent Document 3: US Patent Publication No. 4056530

Patent Document 4: German Patent Publication No. 44406

Patent Document 5: German Patent Publication No. 890104

Patent Document 6: US Patent Publication No. 4359577

Patent Document 7: Japanese Patent Laid-Open No. SHO-56-141354

DISCLOSURE OF INVENTION

Problems to be Solved by the Invention

The present invention has been done in view of the above-describedproblems in the prior art, and is directed to provide a non-pyrophoricnigrosine dye and a method of production thereof.

Means of Solving the Problems

The present inventors diligently investigated in an attempt to obtain anon-pyrophoric nigrosine, and found that there is a clear correlationbetween the aniline content in nigrosine determined by gaschromatography and the pyrophoric activity thereof.

Nigrosine usually contains aniline as an impurity substance. The presentinventors took note of the relationship between sample injection porttemperature of gas chromatograph apparatus and measured aniline contentin the quantitation of the aniline content in nigrosine by gaschromatography (hereinafter also referred to as “the GC method”). Whenthe sample injection port temperature is not higher than a particulartemperature, substantially no changes are observed in the relationshipbetween sample injection port temperature and the quantitative value ofaniline content in nigrosine, and the aniline occurring as an impuritysubstance (hereinafter referred to as “impurity aniline”) is quantifiednearly as is.

However, when the sample injection port temperature exceeds theaforementioned particular temperature, the quantitative value of anilinecontent in nigrosine increases gradually with the rise in temperature.This is considered to be due to the aniline liberated by partialdecomposition of the nigrosine structure (hereinafter referred to as“free aniline”), and the abundance/scarcity of this free aniline isrecognized as being associated with the high/low pyrophoric activity ofnigrosine. The free aniline content can be calculated by setting asample injection port temperature that exceeds the aforementionedparticular temperature and is appropriate for the production of asufficient amount of free aniline, and subtracting the impurity anilinecontent from the measured aniline content.

In analysis using a gas chromatograph apparatus equipped with aglass-made sample injection port, if the aniline content determined atthe sample injection port temperature of 200° C. (corresponding to theimpurity aniline content) is written as A% by weight, and the anilinecontent determined at the sample injection port temperature of 260° C.(corresponding to the sum of impurity aniline and free aniline contents)is written as B% by weight, the pyrophoric activity decreases as the B−Avalue (corresponding to the free aniline content) decreases; nigrosinedyes wherein B−A≦0.04% by weight are non-pyrophoric. There are noconventional nigrosine dyes wherein B−A≦0.04% by weight.

Accordingly, the nigrosine dye of the present invention is characterizedin that B−A≦0.04% by weight if, in analysis using a gas chromatographapparatus equipped with a glass-made sample injection port, the anilinecontent of the sample determined at the sample injection porttemperature of 200° C. is written as A% by weight, and the anilinecontent of the sample determined at the sample injection porttemperature of 260° C. is written as B% by weight.

A determination of the pyrophoric activity of a nigrosine dye can beconducted according to the specified United Nations recommendations. Theabove-described nigrosine dye of the present invention does not ignitespontaneously or does not self-heat to 200° C. or higher within 24 hoursin a test in which the dye is placed in a cubic container 10 cm inlength of each side and kept at a temperature of 140° C. in accordancewith recommendations by the United Nations “Recommendations on theTRANSPORT OF DANGEROUS GOODS—Manual of Tests and Criteria—Third revisededition (Class 4, Test method for self-heating substances)”.

The present inventors diligently investigated on the cause(s) of theabove-described production of free aniline and the pyrophoric activityin conventional nigrosine dyes, and estimated that the production offree aniline is due to partial decomposition of a nigrosine component,and that the accumulation of heat generated due to partial decompositionof a nigrosine component is a cause of the ignition. The method of thepresent invention of producing a nigrosine dye is intended to produce anigrosine dye that permits a reduction in free aniline content due topartial decomposition of a nigrosine component.

Accordingly, the method of the present invention of producing anigrosine dye is a method of producing the above-described nigrosine dyeof the present invention, which comprises a heat treatment of aneutralized nigrosine base.

Because the nigrosine dye of the present invention is non-pyrophoric, itis very safe and hence easy to handle during storage, transportation andthe like. Additionally, according to the method of the present inventionof producing a nigrosine dye, the non-pyrophoric nigrosine dye of thepresent invention can easily be prepared from a conventional pyrophoricnigrosine dye.

BEST MODES OF EMBODIMENT OF THE INVENTION

Any of more specific methods can be used to determine aniline contentsfor the identification of the nigrosine dye of the present invention aslong as the aniline contained in the nigrosine dye can be quantifiedwith high reproducibility at both glass-made sample injection porttemperatures of 200° C. and 260° C. As an example, the method used todetermine A% by weight and B% by weight (aniline contents determinedusing a gas chromatograph apparatus at glass-made sample injection porttemperatures of 200° C. and 260° C., respectively) in the Examples andComparative Examples below is described below.

To an accurately weighed 0.1 g sample, 9 ml of a 1:1 mixture of ethanoland chloroform and 1 ml of an internal standard solution (0.1 g ofaccurately weighed p-nitrotoluene dissolved in a 1:1 [by volume] mixtureof ethanol and chloroform, and prepared to obtain a total volume of 100ml) are added. This mixture is sonicated for 20 minutes and filteredthrough a filter 0.45 μm in pore diameter, 1 μl (microliter) of thefiltrate is injected to the glass-made sample injection port of thefollowing gas chromatograph apparatus, and the aniline content in thesample is quantified.

The gas chromatography apparatus is shown below.

Gas chromatography: HP5890 (manufactured by Hewlett-Packard Company)

Column: DB-1701 (30 m×0.53 mmI.D.×1.0 μm Film), manufactured by J&WCompany

Oven: 100° C. to 260° C. (10° C./min)

Injection port: Made of glass, 200° C. or 260° C., split ratio 10:1

Detector: Flame ionization detector (FID), 280° C.

Injection volume: 1 μl

Other columns, for example, DB-5 (manufactured by J&W Company) and thelike, may be used instead.

The above-described test for the determination of the pyrophoricactivity of a nigrosine dye, i.e., a test in which the dye is placed ina cubic container 10 cm in length of each side and kept at a temperatureof 140° C. in accordance with the United Nations recommendations“Recommendations on the TRANSPORT OF DANGEROUS GOODS—Manual of Tests andCriteria- Third revised edition (Class 4, Test method for self-heatingsubstances)”, can specifically be conducted as described below. In theExamples and Comparative Examples below, pyrophoric activity wasdetermined by the following pyrophoric activity test.

A sample is filled in a sample container, which is a cube 10 cm inlength of each side made of a stainless steel net having 0.053 mmapertures with its upper portion exposed. The sample-filled samplecontainer is housed in a sample container cover, which is a cube of asize such that the sample container can be accommodated therein, whichcube made of a stainless steel net having 0.595 mm apertures with itsupper portion exposed. The sample container cover is placed at thecenter of the inside of a basket 15 cm in length, 15 cm in width and 25cm in height that is made of a stainless steel net having 0.595 mmapertures, and that is suspended at the center of a thermostaticchamber. Also provided is a thermometer (thermocouple) so that thetemperatures at the center of the sample and inside the thermostaticchamber can be measured. After the temperature inside the thermostaticchamber is set at 140° C., this temperature is maintained for 24 hoursand sample temperature is continuously recorded for 24 hours. Ifignition is observed or sample temperature exceeds 200° C. within 24hours, the test is discontinued at that time. Time from test initiationto either ignition or the reach of a sample temperature of 200° C. istaken as time to ignition.

The nigrosine dye of the present invention can be produced by, forexample, a heat treatment of a pyrophoric nigrosine dye in a solvent.More specifically, the following process can be mentioned.

(1) Heat Treatment of Pyrophoric Nigrosine Dye in the Presence ofOrganic Acid

The nigrosine dye of the present invention can be prepared by a heattreatment of a pyrophoric nigrosine dye in a solvent using an organicacid as a catalyst.

An organic acid having a pKa of not lower than 0 is preferable, withgreater preference given to an organic acid having a pKa of 3 to 5.Particularly preferred is an aliphatic or aromatic carboxylic acidrepresented by Formula (1) below.R—(—COOH)_(n)  Formula (1)

[In Formula (1), R is an aliphatic group or an aromatic group.]

As specific examples of such carboxylic acids, aliphatic carboxylicacids such as formic acid, oxalic acid, acetic acid, propionic acid, andbutyric acid; and aromatic carboxylic acids such as benzoic acid,phthalic acids, benzenetricarboxylic acids, and benzenetetracarboxylicacids can be mentioned.

Using an inorganic acid such as hydrochloric acid or sulfuric acid or arelatively strong organic acid (PKa<0) such as an organic sulfonic acid,it is generally difficult to prepare a non-pyrophoric nigrosine dyewherein B−A≦0.04%.

When an organic acid is used as a catalyst, heat treatment temperatureis preferably not lower than 100° C. and not higher than 250° C., morepreferably not lower than 160° C. and not higher than 220° C.

Heat treatment time varies depending on the nigrosine used as a rawmaterial, and is generally 5 to 48 hours. When the treatment time isextremely short, no satisfactory effect is obtained in some cases.

(2) Heat Treatment of Pyrophoric Nigrosine Dye in the Presence of MetalOxide or Heteropolyacid

The nigrosine dye of the present invention can be prepared by a heattreatment of a pyrophoric nigrosine dye in a solvent using a metal oxideor a heteropolyacid as a catalyst.

The aforementioned metal oxide may be one or two or more selected fromamong silica, alumina, silica alumina, titania, molybdenum oxide (VI),tungsten oxide (VI), and vanadium oxide (V).

Also, the aforementioned heteropolyacid may be one or two or moreselected from among H₃PMo₆W₆O₄₀, H₃PW₁₂O₄₀, H₃PMo₁₂O₄₀, H₃PMo₁₀W₂O₄₀,H₃PMo₁₀V₂O₄₀, H₄SiW12O₄₀, and H₄GeMo₁₂O₄₀.

When a metal oxide or a heteropolyacid is used as a catalyst, heattreatment temperature is preferably not lower than 50° C. and not higherthan 250° C., more preferably not lower than 100° C. and not higher than220° C.

Heat treatment time varies depending on the nigrosine used as a rawmaterial, and is generally 5 to 48 hours. When the treatment time isextremely short, no satisfactory effect is obtained in some cases.

(3) Catalyst-Free Heat Treatment of Pyrophoric Nigrosine Dye

The nigrosine dye of the present invention can also be prepared by acatalyst-free heat treatment of a pyrophoric nigrosine dye in a solvent.

In the case of catalyst-free heat treatment, heating temperature ispreferably not lower than 200° C. and not higher than 300° C., morepreferably not lower than 220° C. and not higher than 250° C.

Heat treatment time varies depending on the nigrosine used as a rawmaterial, and is generally 10 to 60 hours. When the treatment time isextremely short, no satisfactory effect is obtained in some cases.

Although the pyrophoric nigrosine dye (nigrosine) used as the startingmaterial in the above-described processes may be produced by any method,it is desirably a neutralized nigrosine base (specific examples includeC.I. Solvent Black 5, C.I. Solvent Black 7, and derivatives thereof).C.I. Solvent Black 7 is more preferable. A nigrosine dye that ignitesspontaneously or self-heats to 200° C. or higher within 24 hours in thetest in which the sample is placed in a cubic container 10 cm in lengthof each side and kept at a temperature of 140° C. in accordance with theabove-described United Nations recommendations is referred to as apyrophoric nigrosine dye (nigrosine).

Although all of the above-described processes can be conducted without asolvent, it is preferable to use a solvent from the viewpoint ofafter-treatment step convenience and the like.

Although various solvents can be used, such as alcoholic solvents,aromatic solvents, aprotic polar solvents, and amine solvents, aromaticamines are particularly suitable.

As examples of such aromatic amines, aniline, methylaniline,dimethylaniline, ethylaniline, diethylaniline, o-toluidine, m-toluidine,p-toluidine, benzylamine, dibenzylamine, tribenzylamine, diphenylamine,triphenylamine, α-naphthylamine, β-naphthylamine and the like can bementioned. Of these amines, anilines having various substituents orunsubstituted anilines are preferred, with greatest preference given toaniline.

The above-described processes permit heat treatment under pressure.

Also, in all of the above-described processes, treatment temperature canbe increased using an autoclave depending on the solvent for use.

EXAMPLES

The present invention is hereinafter described in more detail by meansof the following examples, which, however, are not to be construed aslimiting the scope of the invention. In the description below, “%” means“% by weight”.

Example 1

Ten grams of acetic acid (pKa=4.56) was added to a mixture of 1.0 kg ofa pyrophoric nigrosine (B−A=0.11%), which was a commercially availableneutralized nigrosine base, and 2.0 kg of aniline, and this solution wasrefluxed with stirring under atmosphere for 20 hours (the solutiontemperature during this operation was about 190° C.). Subsequently, thesolution was allowed to cool to about 30° C., then twice washed with 1 L(liter) of 2% aqueous solution of NaOH, and further washed with water.

The nigrosine dye obtained was dried under reduced pressure and thensubjected to GC analysis; the aniline content A in the case of a sampleinjection port temperature of 200° C. was 0.21% and the aniline contentB in the case of a sample injection port temperature of 260° C. was0.24%, i.e., B−A=0.03%. This nigrosine dye did not ignite even after 24hours in the above-described pyrophoric activity test.

Example 2

When starting materials were treated in the same manner as Example 1,except that reflux treatment time was 30 hours, the aniline content A ofthe nigrosine dye obtained in the case of a sample injection porttemperature of 200° C. was 0.21% and the aniline content B in the caseof a sample injection port temperature of 260° C. was 0.22%, i.e.,B−A=0.01%. This nigrosine dye did not ignite even after 24 hours in theabove-described pyrophoric activity test.

Example 3

When starting materials were treated in the same manner as Example 1,except that 20 g of propionic acid (pKa=4.67) was used in place ofacetic acid, the aniline content A of the nigrosine dye obtained in thecase of a sample injection port temperature of 200° C. was 0.18%, andthe aniline content B in the case of a sample injection port temperatureof 260° C. was 0.20%, i.e., B−A=0.02%. This nigrosine dye did not igniteeven after 24 hours in the above-described pyrophoric activity test.

Example 4

When starting materials were treated in the same manner as Example 1,except that 20 g of benzoic acid (pKa=4.00) was used in place of aceticacid, the aniline content A of the nigrosine dye obtained in the case ofa sample injection port temperature of 200° C. was 0.15%, and theaniline content B in the case of a sample injection port temperature of260° C. was 0.18%, i.e., B−A=0.03%. This nigrosine dye did not igniteeven after 24 hours in the above-described pyrophoric activity test.

Example 5

When starting materials were treated in the same manner as Example 1,except that 40 g of silica gel for column chromatography (manufacturedby Wako Pure Chemical Industries) was used in place of acetic acid andthat silica gel was removed by filtration after refluxing, the anilinecontent A of the nigrosine dye obtained in the case of a sampleinjection port temperature of 200° C. was 0.52%, and the aniline contentB in the case of a sample injection port temperature of 260° C. was0.54%, i.e., B−A=0.02%. This nigrosine dye did not ignite even after 24hours in the above-described pyrophoric activity test.

Example 6

When starting materials were treated in the same manner as Example 5,except that reflux treatment time was 24 hours, the aniline content A ofthe nigrosine dye obtained in the case of a sample injection porttemperature of 200° C. was 0.63% and the aniline content B in the caseof a sample injection port temperature of 260° C. was 0.64%, i.e.,B−A=0.01%. This nigrosine dye did not ignite even after 24 hours in theabove-described pyrophoric activity test.

Example 7

When starting materials were treated in the same manner as Example 5,except that 40 g of Molecular Sieves 4A (silica alumina manufactured byAldrich Company) was used in place of silica gel, the aniline content Aof the nigrosine dye obtained in the case of a sample injection porttemperature of 200° C. was 0.31% and the aniline content B in the caseof a sample injection port temperature of 260° C. was 0.33%, i.e.,B−A=0.02%. This nigrosine dye did not ignite even after 24 hours in theabove-described pyrophoric activity test.

Example 8

When starting materials were treated in the same manner as Example 6,except that 30 g of H₃PMo₆W₆O₄₀ was used in place of silica gel, theaniline content A of the nigrosine dye obtained in the case of a sampleinjection port temperature of 200° C. was 0.11% and the aniline contentB in the case of a sample injection port temperature of 260° C. was0.12%, i.e., B−A=0.01%. This nigrosine dye did not ignite even after 24hours in the above-described pyrophoric activity test.

Example 9

When starting materials were treated in the same manner as Example 5,except that 40 g of H₃PMo₁₀V₂O₄₀ was used in place of silica gel, theaniline content A of the nigrosine dye obtained in the case of a sampleinjection port temperature of 200° C. was 0.11% and the aniline contentB in the case of a sample injection port temperature of 260° C. was0.13%, i.e., B−A=0.02%. This nigrosine dye did not ignite even after 24hours in the above-described pyrophoric activity test.

Example 10

A mixture of 1.0 kg of a pyrophoric nigrosine (B−A=0.11%) and 1.0 kg ofaniline was heated at 220° C. with stirring in an autoclave for 48hours.

The nigrosine dye obtained was dried under reduced pressure and thensubjected to GC analysis; the aniline content A in the case of a GCsample injection port temperature of 200° C. was 0.21% and the anilinecontent B in the case of a sample injection port temperature of 260° C.was 0.25%, i.e., B−A=0.04%. This nigrosine dye did not ignite even after24 hours in the above-described pyrophoric activity test.

Comparative Example 1 (Example 1 shown in Patent Documents 6 and 7)

The mixture of 1.0 kg of a pyrophoric nigrosine and 2.0 kg of aniline,used in Example 1, was sparged with air (500 ml/min) at 75° C. duringvigorous stirring for 16 hours. Subsequently, this solution was drieduntil the total content of volatile substances such as aniline becamenot higher than 1%, and then cooled.

The product obtained was subjected to GC analysis; the aniline content Ain the case of a GC sample injection port temperature of 200° C. was0.51% and the aniline content B in the case of a sample injection porttemperature of 260° C. was 0.63%, i.e., B−A=0.12%. It ignited after 6.6hours when tested using the above-described pyrophoric activity testmethod, although Patent Documents 6 and 7 state that the samplesobtained were non-pyrophoric.

Comparative Example 2 (Example 2 shown in Patent Documents 6 and 7)

The mixture of 1.0 kg of a pyrophoric nigrosine and 2.0 kg of aniline,used in Example 1, was sparged with air (16 l/min [liter/min]) at 50 to55° C. during vigorous stirring for 20 hours. Subsequently, thissolution was dried until the total content of volatile substances suchas aniline became not higher than 1%, and then cooled.

The product obtained was subjected to GC analysis; the aniline content Ain the case of a GC sample injection port temperature of 200° C. was0.44% and the aniline content B in the case of a sample injection porttemperature of 260° C. was 0.53%, i.e., B−A=0.09%. It ignited after 7.0hours when tested using the above-described pyrophoric activity testmethod, although Patent Documents 6 and 7 state that the samplesobtained were non-pyrophoric.

Comparative Example 3

When starting materials were treated in the same manner as Example 1,except that 10 g of hydrochloric acid (pKa=−7.0) was used in place ofacetic acid, the aniline content A of the product obtained in the caseof a sample injection port temperature of 200° C. was 0.47% and theaniline content B in the case of a sample injection port temperature of260° C. was 0.78%, i.e., B−A=0.31%. This sample ignited after 3.5 hours.

Comparative Example 4

When starting materials were treated in the same manner as Example 1,except that 20 g of toluenesulfonic acid (pKa=−6.5) was used in place ofacetic acid, the aniline content A of the product obtained in the caseof a sample injection port temperature of 200° C. was 0.57% and theaniline content B in the case of a sample injection port temperature of260° C. was 0.83%, i.e., B−A=0.26%. This sample ignited after 3.2 hours.

Comparative Example 5

When starting materials were treated in the same manner as Example 1,except that reflux treatment time was extremely shortened (3 hours), theaniline content A of the product obtained in the case of a sampleinjection port temperature of 200° C. was 0.20% and the aniline contentB in the case of a sample injection port temperature of 260° C. was0.26%, i.e., B−A=0.06%. This sample ignited after 13.5 hours.

Comparative Example 6

When starting materials were treated in the same manner as Example 5,except that reflux treatment time was extremely shortened (3 hours), theaniline content A of the product obtained in the case of a sampleinjection port temperature of 200° C. was 0.19% and the aniline contentB in the case of a sample injection port temperature of 260° C. was0.30%, i.e., B−A=0.11%. This sample ignited after 7.0 hours.

The results of the Examples and Comparative Examples are summarized inTable 1. TABLE 1 B − A Time (hr) to Treatment conditions (%) ignitionExample 1 Acetic acid (organic acid) 0.03 >24 20 hours Example 2 Aceticacid (organic acid) 0.01 >24 30 hours Example 3 Propionic acid (organicacid) 0.02 >24 20 hours Example 4 Benzoic acid (organic acid) 0.03 >2420 hours Example 5 Silica gel (metal oxide) 0.02 >24 20 hours Example 6Silica gel (metal oxide) 0.01 >24 24 hours Example 7 Molecular Sieves 4A(metal oxide) 0.02 >24 20 hours Example 8 H₃PMo₆W₆O₄₀ 0.01 >24(heteropolyacid) 24 hours Example 9 H₃PMo₁₀V₂O₄₀ 0.02 >24(heteropolyacid) 20 hours Example 10 Autoclave (catalyst-free) 0.04 >2448 hours Comparative Air sparging (75° C.) 0.12 6.6 Example 1 16 hoursComparative Air sparging (55° C.) 0.09 7.0 Example 2 20 hoursComparative Hydrochloric acid (pKa = −7.0) 0.31 3.5 Example 3 20 hoursComparative Toluenesulfonic acid 0.26 3.2 Example 4 (pKa = −6.5) 20hours Comparative Acetic acid (organic acid) 0.06 13.5 Example 5 3 hoursComparative Silica gel (metal oxide) 0.11 7.0 Example 6 3 hours

1. A nigrosine dye characterized in that B−A≦0.04% by weight if, inanalysis using a gas chromatograph apparatus equipped with a glass-madesample injection port, the aniline content thereof determined at theaforementioned sample injection port temperature of 200° C. is writtenas A% by weight and the aniline content thereof determined at theaforementioned sample injection port temperature of 260° C. is writtenas B% by weight.
 2. The nigrosine dye of claim 1, which does not ignitespontaneously or does not self-heat to 200° C. or higher within 24 hoursin a test in which the dye is placed in a cubic container 10 cm inlength of each side and kept at a temperature of 140° C. in accordancewith the United Nations recommendations “Recommendations on theTRANSPORT OF DANGEROUS GOODS—Manual of Tests and Criteria—Third revisededition (Class 4, Test method for self-heating substances)”.
 3. Thenigrosine dye of claim 1, which is prepared by a heat treatment of aneutralized nigrosine base.
 4. The nigrosine dye of claim 1, which isprepared by a heat treatment of a neutralized nigrosine base in varioussolvents in the presence of an organic acid as a catalyst.
 5. Thenigrosine dye of claim 1, which is prepared by a heat treatment of aneutralized nigrosine base in various solvents in the presence of ametal oxide as a catalyst.
 6. The nigrosine dye of claim 1, which isprepared by a heat treatment of a neutralized nigrosine base in varioussolvents in the presence of a heteropolyacid as a catalyst.
 7. Thenigrosine dye of claim 1, which is prepared by a heat treatment of aneutralized nigrosine base in various solvents without a catalyst. 8.The nigrosine dye of claim 4, said solvent being anilines.
 9. Thenigrosine dye of claim 4, said solvent being aniline.
 10. The nigrosinedye of claim 4, wherein the pKa of said organic acid is not lower than0.
 11. The nigrosine dye of claim 4, wherein the pKa of said organicacid is 3 to
 5. 12. The nigrosine dye of claim 11, wherein said organicacid is an aliphatic or aromatic carboxylic acid represented by Formula(1) below,R—(—COOH)_(n)  Formula (1), in Formula (1), R being an aliphatic groupor an aromatic group.
 13. The nigrosine dye of claim 5, wherein saidmetal oxide is one or two or more selected from among silica, alumina,silica alumina, titania, molybdenum oxide (VI), tungsten oxide (VI), andvanadium oxide (V).
 14. The nigrosine dye of claim 6, wherein saidheteropolyacid is one or two or more selected from among H₃PMo₆W₆O₄₀,H₃PW₁₂O₄₀, H₃PMo₁₂O₄₀, H₃PMo₁₀W₂O₄₀, H₃PMo₁₀V₂O₄₀, H₄SiW₁₂O₄₀, andH₄GeMo₁₂O₄₀.
 15. The nigrosine dye of claim 4, wherein the temperaturefor said heat treatment is not lower than 100 and not higher than 250.16. The nigrosine dye of claim 5, wherein the temperature for said heattreatment is not lower than 50 and not higher than
 250. 17. Thenigrosine dye of claim 7, wherein the temperature for said heattreatment is not lower than 200 and not higher than
 300. 18. Thenigrosine dye of claim 3, wherein said heat treatment is conducted underpressure.
 19. A method of producing the nigrosine dye of claim 1, whichcomprises a heat treatment of a neutralized nigrosine base.
 20. Themethod of producing a nigrosine dye of claim 19, wherein said heattreatment is conducted in a solvent.
 21. The method of producing anigrosine dye of claim 19, wherein said heat treatment is conducted inthe presence of an organic acid, a metal oxide, or a heteropolyacid as acatalyst.
 22. The method of producing a nigrosine dye of claim 19,wherein said heat treatment is conducted under pressure.
 23. The methodof producing a nigrosine dye of claim 19, said solvent being aniline.